This is incorrect. You may have gotten the correct for this instance, because there's only one tag in the tags table. However, you counted all the statuses with tags, without differentiating between tags. You should have counted the different tags. Also, to more deliberately answer the question, you should have used a 'limit' clause to limit the number of records returned to one, the maximum. The following example would work:

SELECT tags.tag  FROM tags INNER JOIN statuses_tags ON tags.id = statuses_tags.tag_id INNER JOIN statuses ON statuses_tags.status_id = statuses.id GROUP BY statuses_tags.tag_id ORDER BY COUNT(statuses_tags.tag_id) DESC LIMIT 1;